libbinder: allow externally created connections

We want to allow:
- VM B process S has server using RpcServer normally
- VM A process V calls socket() and connect()
- VM A process V passes connected socket to VM A process C
- VM A process C talks to S over socket directly

Where:
- V = virtmanager process
- S = server process (e.g. compOS)
- C = client process (e.g. installd)

This way, within a VM, only one process needs permissions to connect to
services, and using this service, we can control access to VM services.

Bug: 193801719
Test: binderRpcTest
Change-Id: I5af60fd24354bce21d79676231172b6357b0c1a3